An antibiotic is a natural or synthetic compound that can kill or inhibit microbial pathogens.  Anitibiotics which kill are termed cidal, and those which inhibit are called static.  Static antibiotics inhibit the growth of microbes, which allows the host immune response to kill the pathogen.

Antibiotics must have selective toxicity, that is they must be specific for the pathogen and not harm the host.  The more selective an antibiotic, the more advantagous its use.

Antibiotics work in a variety of ways.  Some, like penicillin, destroy bacterial membranes.  Others, such as sulfa drugs, inhibit cellular processes specific to the pathogen.  They inhibit aspects of protein and nucleic acid synthesis, destrupt cell membranes, and interfere with metabolic pathways.

Some are highly specific for particular pathogens, and others have more general applicability.  They can act against bacteria, fungi, protozoa, or viruses, though the use of the word antibiotic is most often in reference to antibacterial drugs.  They greatest number of antibiotics have been developed to fight bacteria, because they are the easiest to target.

The United States produces millions of pounds of antibiotics a year, which generate billions of dollars in revenues.  Antibiotics have several uses.  These include prescription by doctors to treat bacterial infections and usage in animal feed to prevent infection and promote growth.  The greatest obstacle to the efficacy of antibiotics is the ability of pathogens to become resistant to them.
 

Information from:

Prescott, L.M., J.P. Harley, and D.A. Klein. Microbiology. 1999. Fourth edition, Wm. C. Brown Publishers, Dubuque, IA.